Senile dementia of the Alzheimer type (SDAT) is the most common disabling neurologic disorder of late life, a major cause of death and hospitalization. It is the leading indication for nursing home admission. The rising prevalence of SDAT will shortly consume most of the health care budget for the elderly in the U.S. and may disrupt organization of acute hospital care unless major insights permit treatment or prevention. Early diagnosis in SDAT is essential for etiologic studies, as well as intervention trials and counselling. Recall of data available only to the patient, cooperation in searching evaluation, avoidance of bias due to institutionalization, moveout or death, and separation of disease effects from sequelae depend upon early diagnosis, as may identification of toxic or infectious causes. The relevance of onset age to the course and manifestations of Alzheimer's disease and the relation of SDAT to normal aging will resist study until early diagnosis become practical. There is no accepted positive diagnostic technique, save hazardous and often inconclusive brain biopsy. Present exclusionary (negative) techniques are expensive, subjective, nonstandard and insensitive. Early prevalent and incident SDAT cases will be compared with controls to develop a diagnostic profile of performance in multiple neuropsychologic spheres, including memory, language and visuospatial performance. Profiles will be developed to separate SDAT from normal aging and depression. Profiles will be tested across disease stages among and (longitudinally) within individuals. An objective diagnostic profile based on data obtained in life by nonphysicians will advance etiologic research. It may ultimately save expense in evaluating typical SDAT cases and supplement subjective medical assessment in complex cases. In the process of identifying the sample for longitudinal neuropsychologic study and confirming diagnosis in other samples, codification, extension of the range and refinement of a sensitive, economical exclusionary approach to diagnosis will continue. Normal aging and SDAT will be compared with regard to patterns of alteration in cerebral function, and the relative rates of change in specific components of higher function. Changes in global cognitive and social function over time will be evaluated to determine whether they suggest a threshold effect.